1. Field of the Invention
The present invention relates to fired iron ore pellets having precipitated slag phases of at least two different chemical compositions, and which provided excellent reducibility for use in the manufacture of pig iron in a blast furnace and to a process for producing the same.
2. Description of the Prior Art
Recently, a trend has developed for increasing the yield of fired iron ore pellets by utilizing low grade and fine iron ore produced during mining and screening processes. (Fired iron ore pellets will simply be referred to as pellets or iron-ore pellets, hereinafter.)
In addition, iron ore can be easily pelletized into pellets of the desired sizes which afford uniform properties. The iron ore pellets have the feature that their properties can be varied by simply changing the conditions of production of the pellets. For these reasons, iron ore pellets find wide use in blast furnaces.
Because of the manner in which pelletizers operate, iron ore pellets in the past have been produced in spherical form or the equivalent, so that the ratio of the outer surface area of the pellet to its volume is minimized. This is apparently undesirable from the viewpoint of the reducibility of the pellets, because the larger the area of contact between the reducing gases and the materials to be reduced, the better the reducibility of the pellets. Thus, in order to improve the reducibility of the pellets, it has been proposed to increase the porosity of the pellets, particularly the open-porosity thereof.
In general, iron ore pellets have mirco-pores which provide a relatively high degree of porosity to the pellets of about 30%. Consequently, the pellets exhibit good reducing properties in reduction processes at temperatures less than 900.degree. C. Furthermore, depending upon the conditions of production of the pellets, pellets sometimes result which suffer from low porosity and, hence, possess decreased reducibility, although the pellets possess the desired compressive strength, tumbler index, softening properties, swelling properties, and the like.
In addition, another problem arises in that at high-temperature reduction at temperatures of over 1000.degree. C., the micro-pores of the pellets are closed or clogged by low-melting slag because of the sintering reaction of pellet forming grains, thus leading to retardation of reduction.
In order to avoid these shortcoming of the pellets, one solution has been to increase the outer surface areas of the pellets. However, attempts to reduce the grain size of the pellets has led to a lowering of the permeability of the charge in a blast furnace and sticking of the pellets into large-sized lumps which impairs the satisfactory operation of a blast furnace. For this reason, this approach to the problem has been considered to be unsatisfactory.
Another one of the shortcomings of the prior art pellets stems from the physical properties of the pellets, i.e. the spherical shape thereof, in that the spherical shape of the pellets exerts a considerably adverse effect on the operation of the blast furnace. The shortcomings of the conventional pellets will be described in more detail by reference to FIGS. 1 and 2 hereinafter.
In the operation of a blast furnace, as shown in FIG. 1, spherical pellets of sizes ranging from 5 to 20 mm in diameter and in a preweighed amount are alternately charged with batches of coke which serve as a reducing agent through charging portion 2 of blast furnace 1 with the result that pellet layers (PL) and coke layers (CL) are formed within the furnace in a layer-by-layer relationship, i.e. with one layer on top of another. As a result of this method of charging the furnace, the surface contour of the top layer, in general, assumes a "V" shape in its cross section, i.e. the center portion of the top layer is the lowest part of the uppermost layer and then the layer slopes upwards towards the peripheral portion of the layer. It is desirable to provide uniform piles of pellet layers (PL) and coke layers (CL) in the furnace which have a minimized variation in thickness in the radial direction of the furnace. However, in practice, this results in a failure to achieve a successful operation, because of the marked difference in physical properties between coke and pellets. As shown in FIG. 2, when pellets (P) are charged to a furnace onto the top layer of coke (CL) within a furnace, the pellets tend to flow from the outer periphery of the furnace towards its center so that the thickness (t1) of the central portion of the pellet layer (PL) is thicker in comparison to the thickness (t2) of the peripheral portion of the pellet layer, thus providing a lack of uniformity in the thickness of the pellet layer in the radial direction of the furnace. When coke is further charged onto the top pellet layer (PL), the amount of coke which tends to flow from the peripheral portion of the furnace towards its center is reduced, relative to the flow pattern of pellets because the coke particles are of a larger size than the pellets. As a result, the thickness (t1) of the central portion of the coke layer (CL) is substantially decreased in comparison to the thickness (t2) of the peripheral portion of the coke layer (CL), thereby providing a lack of uniformity in thickness of coke layer in the radial direction of the furnace. In this manner, the pellet layers and the coke layers are laid one on top of another within the furnace, resulting in a distribution in which the pellets tend to segregate in the central portion of the furnace while the coke tends to segregate toward the peripheral portion of the furnace, as shown in FIG. 2. As a result, the speed of gases flowing from the base of the furnace upward through the peripheral portion of the charge in the furnace is faster than the speed of gases flowing upward through the central portion of the charge in the furnace, as shown by the arrows in FIG. 2. It follows from this that the temperature in the peripheral portion of the charge in the furnace is higher than the temperature in the central portion of the charge, so that the amount of reducing gases which is produced is increased in the peripheral portions of the charge relative to the amount of gases produced in the center of the charge. Thus, reduction of the pellets occurs unevenly across the bed in the furnace with a greater extent of reduction occurring in the periphery of the portion thereof.
The amount of a charge which flows into the central portion of the furnace is largely dependent upon the so-called "angle of repose" of the charge therein. Table 1 shows the relationship between the angles of repose and inclined angles of layers in a furnace. The angle of repose of pellets is smaller than that of coke, and such a difference in angle of repose is responsible for the lack of uniformity in thickness of the layers in a furnace. On the other hand, the angle of repose of sintered ore is on the same order of that of coke, so that the flow pattern of conventional ore pellets does not occur with sintered ore and a uniform distribution of thickness of deposited layers may be readily achieved. This may be attributed to the fact that pellets are of a spherical form approximating a true sphere and, therefore, the pellets provide smooth contacting surfaces which give rise to extremely low contact frictional resistance between pellets in comparison to sintered ore or coke of a complex, irregular shape.
TABLE 1 ______________________________________ Charge Angle of repose inclined angle of layer ______________________________________ Pellets 25 to 28.degree. 20 to 26.degree. sintered ore 31 to 34.degree. 29 to 31.degree. coke 30 to 35.degree. 33 to 38.degree. ______________________________________
As has been described, the lack of uniformity in flow of the prior art pellets towards the center of a furnace and the distribution pattern of thickness thereof leads to a disturbance in the uniformity of the coke layers and a biased flow of reducing gases towards the peripheral portion of the charge in an inconsistent manner, as well as to unbalanced descent of charge into the furnace. This impairs the reducing reaction within the furnace and lowers the operational efficiency thereof. In addition, even after the pellets are charged in the furnace, vibrations or irregular movements of the pellets occur because of the flow of gas, so that the pellets tend to be mixed with an adjoining coke layer, thus causing an uneven thickness in the coke layers, which exerts an adverse influence on the permeability of the gases in the furnace and on the reactivity of the coke. The results are an increased coke ratio and lowered yield of production.
It is also known that the reduction of a pellet proceeds from its peripheral portions towards its central portion in a topochemical fashion. Thus, in the reduction reaction, a closely packed metallic iron layer, i.e. the products of reduction, is formed in the peripheral portions of the pellet at a high-temperature zone of a furnace, so that the ingress of reducing gases into the interior of the pellet is hindered, and hence, an unreacted core is likely to remain in the interior of the pellet. This drawback as well is attributable to the spherical shape of the ore pellet. Simultaneously, with the formation of an unreacted core in the pellet, the pellets soften and a lowering of the melting down temperatures of the pellets occurs with the result that the pellets become sticky. In addition, because of the spherical shape of the pellets, very closely packed layers of pellets occur within the furnace with the accompanying reduction in voids in the pellet layers. As a result, the sticking phenomenon of pellets is further promoted. Apparently, the sticky condition of the pellets detrimentally affects the permeability of the reducing gases in the pellets and consequently reduces the operational efficiency of the furnace.
A need, therefore, continues to exist for a method by which iron ore pellets can be more evenly and thoroughly reduced in a blast furnace.